The sanitization of a standing or recirculating aqueous medium typically involves introducing a hypohalite, e.g., a hypochlorite, anion donor material, such as a halogen or halogen-containing material, e.g., calcium hypochlorite, into the aqueous medium so as to establish a desired level, e.g., a sanitizing amount, of free available halogen, e.g., free available chlorine (FAC), within the aqueous medium. The presence of free available chlorine within the aqueous medium, e.g., a swimming pool, serves to eradicate or control deleterious amounts of pathogenic bacteria, algae, etc that are present in the aqueous medium. As an oxidizer, hypochlorous acid can also remove nutrients from the water, thus providing indirect protection against microbial infestation. Sanitation of water contacted by humans and animals is required because exposure to unsanitized or inadequately sanitized water that contains deleterious amounts of pathogenic bacteria, viruses, protozoa, etc can lead to the development of infection or disease.
Free available chlorine can be established in an aqueous medium by adding regularly a source of hypochlorous acid (HOCl) or hypochlorite anion (ClO−) to the water comprising the aqueous medium. Recreational bodies of water, e.g., swimming pools, hot tubs, spas, etc, typically are treated so as to contain FAC in an amount of from 1 to 3 parts per million parts (ppm). Water having a FAC content in amounts of greater than 10 ppm can be used to sanitize surfaces or articles to which it is applied.
Hypochlorous acid or hypochlorite anion is commonly introduced into water by passing the water, or a portion thereof, through a chlorination unit, which contains a donor source of the hypochlorous acid or hypochlorite anion. Other means include introducing chlorine directly into the water or adding the hypochlorous acid donor material directly into the body of water to be treated. A common donor source of hypochlorous acid or hypochlorite anion is calcium hypochlorite. Solid forms of calcium hypochlorite, e.g., granular calcium hypochlorite, can be added directly into the water to be treated, or placed in a chlorination unit, e.g., as granules or tablets. When contacted with water introduced into the chlorination unit, the calcium hypochlorite dissolves, thereby providing an aqueous solution comprising hypochlorite anion, which solution is added to the water to be treated.
In the case of a standing or recirculating body of water, e.g., swimming pools, periodic batch additions of higher levels of hypochlorite anion can be made to the body of water in addition to the relatively steady and lower level additions described previously. Such batch additions of higher levels of hypochlorite anion are commonly referred to as a “shock treatment” or as a “super chlorination” and are made on a periodic basis, e.g., once a week or once a month. Typically, the purpose of a shock treatment is to briefly increase the FAC of the body of water, e.g., by 5 to 20 ppm, to consume accumulated organic material, destroy chloramines and/or control algae blooms. A shock treatment is administered by, for example, preparing a concentrated aqueous solution of calcium hypochlorite from granulated calcium hypochlorite, and then adding this solution to the body of water, or distributing, e.g., broadcasting, granulated calcium hypochlorite directly over the surface of the body of water.
Depending on the volume of water that is to be sanitized, the amount of calcium hypochlorite that is stored on site for use in a chlorination unit and/or for shock treatments can be large. Calcium hypochlorite is a material that can cause or enhance the combustion of organic materials by providing oxygen for combustion, e.g., it serves as an oxidizer.
In accordance with Title 49, Code of Federal Regulations, part 173, section 127, paragraph (a), subparagraph (1), [49 CFR §173.127(a)(1)], calcium hypochlorite is categorized as a Division 5.1 oxidizer. It is classified as a Packing Group II oxidizer material [49 CFR §173.127(b)(ii)]. The transport of a material categorized as a Division 5.1 oxidizer requires the use of special precautions, which can include the use of special containers. The storage of NFPA (National Fire Protection Association) class 3 oxidizers (which UN Group II oxidizers will commonly be) can require separate free standing storage facilities and/or special sprinkler systems. Such special provisions for storage and handling can result in increased costs for these activities.
In addition to the special handling associated with the storage of calcium hypochlorite due to its oxidizer classification, there is also the matter of its storage stability, e.g., shelf life. In particular, loss of FAC upon storage of calcium hypochlorite, though much lower than with sodium hypochlorite, can be significant under some circumstances. Use of low assay calcium hypochlorite can result in inadequate sanitization of water to which it is added. Further, the loss of FAC upon storage can be accelerated if the calcium hypochlorite is formulated with materials that tend to enhance the loss of FAC during storage. For example, calcium hypochlorite compositions prepared from an admixture of calcium hypochlorite and boric acid can lose an appreciable amount of free available chlorine upon storage.
It is, therefore, desirable to develop a solid calcium hypochlorite composition that is not classified as a Packing Group I or Packing Group II Division 5.1 oxidizer, and which, in comparison to calcium hypochlorite that has not been blended with other additives, has a favorable stability with regard to FAC loss upon storage. It is also desirable that such a newly developed solid calcium hypochlorite composition have an FAC content at least sufficient to allow its use in the batch and/or continuous sanitization of an aqueous stream or body of water.